i.: :-:::- '.LI?.}' AT A : Z:H:V::IZA 



- ... - .-- ___ ....__--.. 

 -. . . - - . -..----. . -. -. -. ' 



antHrfrfT^ 1 frbfiy or tobe-feet, ca 



:--: i--v-i: :-..-. ,_:-. v.t ;_;:-: 



cral areas, and can be again 

 ictiacted. By mejns of 



\:.^'.^.\> .'.::.'.''.:. .:-. ^~~~.~.~i. 

 die tube-feet being capable 

 of protrusion to a length 



lEPCSttd* tfKlFl f fr^fTt OK toO 



longest spines of die body. 

 The mechanism by which 

 the tube-feet are protruded 

 and retracted is as follows : 

 Each tube foot, shortly after 

 its origin, gives rise to a 

 secondary lateral branch, 

 which terminates in a vesi- 

 cle. These vesicles or "am- 

 puH3e w (D) are provided with 

 circular muscular fibres, by 

 the contraction of which 

 their contained fluid is forc- 

 ed into the tube-feet, which 

 are thus protruded. Retrac- 

 tion of the ambulacra! tubes 

 is effected by proper mns- 



UK 



i: :lit ^.ll ::' 



Pofian 



s. Only die 

 sack are shown 



tobe4eet(/X with their 



* 



of four of the 



and a few of 



cular fibres of their own, 

 which expd again the fluid 

 which has been forced into 

 them by the vesicles. The 

 walls of the stone-canal are 

 strengthened by calcareous 

 deposits ; and die termina- 

 tions of the tube-feet con- 

 tain in many forms a cal- 

 careous rosette, often with 

 a calcareous ring below it, whilst the walls of the tube-feet are 

 furnished with calcareous spicules. 



The total area over which the tube-feet can be protruded 

 depends upon the extent to which the "ambulacra!" or " pori- 

 ferous " zones of the test are developed. In the typical or 

 " Regular " Sea-urchins, the ambulacra! areas are "perfect," 

 and extend from pole to pole; whereas in the so-called 

 "Irregular" Urchins (such as the Heart-urchins and Cake- 



side of one of the radiating canals. 



